Arthritis is the most common chronic disease in the world There are many causes of arthritis, and the causes of joint damage are also different. Currently, Tofacitinib (CP-690550) is a novel oral JAK (Janus Kinase) pathway inhibitor developed by Pfizer Inc. Tofacitinib is a first-in-class drug developed for rheumatoid arthritis treatment. Based on the structure of tofacitinib, a series of JAK inhibitor compounds, which are active in vitro and in vivo and highly absorbable, have been developed. See WO2013091539. The compounds disclosed in WO2013091539 were screened and prepared as salts in which (3aR,5s,6aS)—N-(3-methoxyl-1,2,4-thiadiazole-5-yl)-5-(methyl(7H-pyrrolo[2,3-d]pyrimidine-4-yl)amino)hexahydrocyclopenta[c]pyrrole-2(1H)-formamide bisulfate of formula (I) was obtained, and its preparation method was disclosed in PCT Patent Application No. PCT/CN2014/076794 (an application previously filed by the applicant).

It is well known that the crystal structure of a pharmaceutically active ingredient often affects the chemical stability of the drug. Different crystallization conditions and storage conditions can lead to changes in the crystal structure of the compound, and sometimes the accompanying production of other crystal forms. The compound of formula (I) was dissolved in methanol, and then part of the solvent was evaporated to precipitate a crystal, which was named crystal form I. See Chinese Patent Application No. 201410529863.8. In the subsequent study, we surprisingly found that the compound also has another crystal form. When the salt is refluxed in a multisolvent system to produce a crystal transformation, or crystallized in a variety of solvents, another crystal is obtained, which is defined as crystal form II herein. Studies on the factors that influence crystal forms and the specific stability tests showed that the crystal stability of crystal form II is good, and this crystal form is the most easily obtained stable crystal form of the compound. Its preparation process is controllable and repeatable, thus it is more suitable for industrial production.